In inductive energy transmission, a spatial separation is fundamentally provided between a primary coil and a secondary coil. This spatial separation is primarily due to a freedom of positioning the contactless inductive energy transmission process or charging. Said spatial separation, however, may result in disadvantages especially for electromagnetic compatibility (EMC). This is essentially the result of the open type of construction of the transformer with the primary coil and the secondary coil. This is to be seen in contrast to a conventional transformer, which is well encapsulated and whose primary and secondary coils are situated as close to each other as possible.
This open type of construction results in the problem explained in the following with reference to FIG. 1. FIG. 1 shows a fundamental situational equivalent circuit diagram of the mentioned conventional “open construction type” during an EMC test scenario. A primary side resonant circuit may be seen that is formed by a primary coil 10, whose winding produces a resonant inductance, and a first capacitor 15, which produces a resonant capacitance. First capacitor 15 is connected to a local ground potential 16 of inductive energy transmission system 100. The primary-side resonant circuit is supplied by an exciting electrical useful voltage source UEx. The exciting useful voltage source UEx provides an electrical useful signal for the inductive energy transmission system 100. The useful voltage source UEx is overlaid with an electrical noise voltage source US, which represents all noise signals of the inductive energy transmission system 100. A noise signal in the sense of the present invention is to be understood primarily as common-mode noise. Generally, noise signals are understood both as common-mode noise and as differential-mode noise.
In terms of circuit technology, the two coils 10 and 20 respectively also form an electrode. With the primary coil and the secondary coil 10, 20, two electrodes are thus facing each other, between which a parasitic mutual capacitance 11 is situated. The parasitic mutual capacitance 11 is low primarily due to the wall thicknesses of the two housings of the coils 10, 20, common values of mutual capacitance 11 being between approximately 5 pF and approximately 25 pF. The two electrodes of coils 10, 20 respectively form a parasitic ground capacitance 12, 13 with respect to ground. Usual values for ground capacitances 12, 13 are in the range of approximately 20 pF to approximately 40 pF.
The secondary side of inductive energy transmission system 100 has an electronic load that is represented by a resistor 60 that is connected in parallel to secondary coil 20. The load may be for example an ohmic consumer (e.g. an incandescent lamp) or, as is usual in inductive chargers, an accumulator. The entire secondary system furthermore has another ground capacitance 14, which is connected to load 60 and to the ground potential.
FIG. 1 further shows a measuring resistor 40 of a radio measuring receiver that is required for EMC measurement. Measuring resistor 40 may be configured as a 50 ohm terminal resistor. The electrical voltage dropping on measuring resistor 40 is designated by Um and represents the electromagnetic noises on the radio measuring receiver.
The circuit configuration from FIG. 1 thus shows that the above-mentioned common-mode noise may cause electrical displacement currents that flow via ground capacitors 12, 13, 14 to ground. This is indicated by three closed circuits via ground capacitors 12, 13, 14 that are represented as dashed lines. The displacement currents flow on the ground via the radio measuring receiver back to noise voltage source US. The equivalent circuit diagram from FIG. 1 thus illustrates a coupling path of the noise via the ground capacitors 12, 13, 14.